the actually reason behind it is that its easier to maintain
use to with nt 4 server and client shared same code same with windows 2000 and windows server 2000
however office and home pcs were different codebases so with windows xp they merged these codebases and had planed to make the server client and home user pc share same codebase
however what happened was mid way though releasing server 2003 there was major security bugs discovered and they changed there plans and the codebase begen to deverge with windows xp service pack 2 while the server edition had been updated for more secuity
and then vista was released microsoft had planed to try and finaly get the codebases in sync agin so that they only had to fix bugs once for server and client os
service pack 1 main goal was to get vista and server 2008 sharing the same code
so in my opinion we only need one forum for server 2008 and vista 98 percent of the hotfixes will be compatible between them with the exception of some files like media center for client and active directory for server which when you attempt to appley should say this update does not apply to your system